In electronic timepieces having large hands (especially, the second hand) to improve visibility, a problem arises in that if the timepiece is dropped or is hit against something, the phase of the rotor errantly shifts and the displayed time becomes incorrect due to the impact. Patent Document 1 discloses a method to cope with the problem, where impact-induced rotor vibration is detected; if it is determined that an impact has occurred, an electric current is immediately and forcibly caused to flow in a coil, applying a braking force to the rotor, whereby the display of an incorrect time due to the impact is prevented. This method will hereinafter be referred to simply as “electromagnetic braking scheme”.
The conventional art disclosed in Patent Document 1 will be described briefly with reference to FIGS. 21 and 22. FIG. 21 is a block diagram of a configuration of an example of the conventional art. FIG. 22 is a diagram of a waveform output by an electronic timepiece of the example. In FIG. 21, “1” denotes a stepper motor that is configured by a rotor 10 and a coil 13; “101” denotes a driving pulse generating circuit that generates a driving pulse Pa that drives the stepper motor 1; “102” denotes a locking pulse generating circuit that generates a locking pulse PL to brake and control the rotor 10 when the stepper motor 1 is subjected to an impact; “103” denotes a pulse selecting circuit that selects the driving pulse Pa generated by the driving pulse generating circuit 101 or the locking pulse PL generated by the locking pulse generating circuit 102; “108” denotes a driver circuit that outputs to the coil 13, the pulse selected by the pulse selecting circuit 103; and “104” denotes an impact detecting circuit that detects occurrence of an impact using a counter-electromotive current that is generated in the coil 13 by a vibration of the rotor 10.
Operations of the circuits will be described. As depicted in FIG. 22, the driving pulse Pa that is output from the driving pulse generating circuit 101 at a timing s1 that is at the beginning of a second is output from “O1” of the coil 13 through the pulse selecting circuit 103 and the driver circuit 108 and causes the rotor 10 to rotate by 180 degrees. A given time period during which the vibration of the rotor 10 caused by the driving is presumed to come to an end is provided as a dead time period T1 during which no impact detection is executed and thereafter, the operation moves to that in an impact detection time period T2 during which an impact is detected. During the impact detection time period T2, the impact detecting circuit 104 periodically detects, using an impact detection signal g, a counter electro-motive voltage generated by an impact. When a counter-electromotive voltage is generated by an impact G within the time period of the impact detection time period T2, the impact detecting circuit 104 immediately controls the locking pulse generating circuit 102 and the pulse selecting circuit 103 to output the locking pulse PL, and brakes and controls the rotor 10 using the locking pulse PL output from O1 of the coil 13. After the locking pulse PL is output, the dead time period T1 is provided during which a vibration caused by the locking pulse PL comes to an end. Thereafter, the operation moves to that in the impact detection time period T2 during which an impact is again detected.
The locking pulse PL is output in the same phase (O1) as that of the driving pulse Pa. Usually, the rotor 10 is rotated by 180 degrees by the driving pulse Pa and therefore, the locking pulse PL output thereafter is output in the phase for the rotor 10 not to rotate. Therefore, no case is present where the rotor 10 is rotated by the locking pulse PL and thereby, the displayed time becomes wrong.
Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2005-172677
Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2000-75063
Patent Document 3: International Publication No. 95/27926